1. Field of the Invention
This invention relates in general to well logging systems and in particular to borehole televiewer systems which permit the wall of a borehole to be scanned in a manner which may be utilized to produce a visual indication of the borehole wall.
2. Description of tho Prior Art
Borehole televiewer systems are well known in the prior art. One example of such a system is disclosed in U.S. Pat. No. 3,369,626, wherein the walls of a borehole are scanned periodically with acoustic energy for obtaining information of interest. In one embodiment, a single transducer which acts both as a transmitter and receiver, is rotated in the borehole and periodically actuated to produce acoustic pulses which are applied to the borehole wall. Reflected energy is then detected by the transducer between acoustic pulses and converted into reflection signals.
A beam sweep is produced once for each transducer rotation signal and applied to the horizontal deflection plates of an oscilloscope to sweep an electron beam along a path horizontally across the screen of the scope. The reflection signals are then employed to intensity modulate the electron beam of the oscilloscope as the beam is swept across the screen of the scope.
In one embodiment of the aforementioned United States patent, a sawtooth waveform is produced during each transducer operating period which encompasses the expected arrival of the reflection signal. Each of these sawtooth waveforms is applied to both deflection plates of the oscilloscope to deflect the beam vertically from its otherwise horizontal path. The reflected energy is detected at a time which is a function of the time or distance between the transducer and the wall of the borehole. Thus, the reflection signals will intensify the electron beam during different time periods of each sawtooth waveform and at different vertically displaced positions. Hence, a horizontal trace is produced across the screen which has vertical excursions or deviations which are a function of the time or distance between the transducer and the wall of the borehole.
U.S. Pat. No. 3,550,075, discloses a technique and system which converts the reflection signals to pulses having heights which are a function of the time or distance between the transducer and the borehole wall. These pulses are employed to intensify the electron beam as it is swept across the screen of an oscilloscope during each downhole cycle. A resulting trace pattern is formed which varies in intensity or tones as a function of the time or distance between the transducer and the borehole wall. The trace patterns obtained by this method may be recorded in a side-by-side relationship to form a display which gives information about the configuration of the borehole wall.
The above-referenced patents both disclose so-called borehole televiewer systems which are typical of the commercially available systems currently employed. Such systems suffer certain deficiencies which prevent accurate scanning of the borehole wall. For example, the rotating or scanning transducer in each of these systems is generally provided in a fixed sleeve or cavity within the tool, which is then filled with a special hydraulic fluid. This fluid is selected to minimize the interface between the inside of the sleeve and the borehole fluids; however, the interface present still results in extraneous signal reflections and resonances which may cause parasitic components in the received signal. These parasitic components, when superimposed on the measured signal, will adversely affect the processing of the received signal.
A second deficiency in known borehole televiewer systems is the utilization of a fixed speed rotating member to drive the scanning transducer. Generally, a rotating member or motor is selected which operates at a selected rotation speed which will provide the degree of accuracy desired when the tool is moved axially through the borehole at a preselected speed. However, it is not always possible to log a borehole at a particular speed and the accuracy of the resultant measurements will suffer as the logging speed deviates from the design speed for a particular tool.
A final design deficiency of known borehole televiewers systems is the inability of the field engineer to rapidly and efficiently repair or replace the measurement transducer in a particular tool. This is due to the aforementioned method of mounting the rotating transducer in a sealed cavity filled with hydraulic fluids. One advantage of a design which will permit the exchange of the transducer head is the ability of the engineer to utilize a transducer which is particularly adapted to operate in a borehole of known diameter.
It should therefore be apparent that a need exists for an improved borehole televiewer system which permits a highly accurate scan of a borehole wall to be accomplished.